1) Field of the Invention
The present invention relates to a technology for developing a latent image carried on an image carrier with a two-component developer.
2) Description of the Related Art
A developing device is widely used in an image forming apparatus for developing an electrostatic latent image using a two-component developer (hereinafter, “developer”), i.e., a developer that contains a toner and a magnetic carrier. The developer is accommodated in a developer accommodating unit and is subjected to triboelectric charging. A photosensitive body serves as an image carrier for carrying an electrostatic latent image. The developer is carried onto a surface of a developer carrier, which has a non-magnetic sleeve and a magnetic field generating unit inside, so that the developer is conveyed to a developing region opposed to the photosensitive body. In the developing region, an electric field corresponding to an image is formed between the electrostatic latent image on the photosensitive body and the sleeve, and toner in the developer on the sleeve is attached to the photosensitive body by the electric field so that development is performed to form an image.
To perform an appropriate developing, an appropriate amount of the developer is conveyed to the developing region. A doctor blade that is provided opposed to the sleeve, with a predetermined clearance, as a means that allows only a desired amount of the developer to be carried on the sleeve and conveyed to the developing region. Precisely, if there is extra developer on the sleeve, the doctor blade scraps it. However, when the doctor blade scraps the developer, it exerts a mechanical stress on both the developers, i.e., the developer that is scrapped and that is allowed to pass. If the developer receives the mechanical stress repeatedly, the quality of the developer deteriorates. Precisely, crushing of toner and carrier, burring/separating of additives into/from toner, or toner component spent to carrier occurs due to the mechanical stress. Such a developer is difficult to electrically charge uniformly so that drawbacks such as degradation of graininess over time, a background fogging, and toner scattering occur.
In particular, when toner in which wax is dispersed for ensuring a fusing releaseability corresponding to oilless fusing is used as the toner, the wax component with the same polarity as the toner oozes on the toner surface due to heat generated by the mechanical stress so that the carrier surface is coated with a film of the wax. If there is a wax film on the carrier, the carrier can not be charged properly so that the amount of charged toner becomes less than desired. This causes problems such as toner scattering or background fogging. Various proposals have been made to overcome this problem.
Japanese Patent Application Laid-Open No. H8-278695, for example, discloses a developing device in which a diametrical magnetic flux density of a magnetic pole inside a developer carrier at a position opposed to a developer restricting member is set in the range of 5 millitesla to 35 millitesla. Because the diametrical magnetic flux density is less, lesser mechanical stress is exerted on the developer at the developer restricting member.
Japanese Patent Application Laid-Open No. H10-333431 discloses a developing device in which a plurality of developer restricting members are arranged along a rotational direction of a developer carrier, the developer restricting member positioned at the most downstream side is formed as a magnetic member, and a center portion of a trimming magnetic pole of fixed magnets positioned inside the developer carrier before a developing region is arranged between the developer restricting members of the most downstream side and the most upstream side. In this developing device, the developer on the developer carrier sequentially forms layers while passing through the developer restricting members. Since a magnetic force of the trimming magnetic pole is arranged so as to act mainly between the magnetic member of the most downstream developing restricting member and itself, a magnetically restricting force acts on a gap portion of the most downstream developer restricting member so that the developer passes through the gap portion in a spiked state. Therefore, even if the most downstream developer restricting member has a board gap, it is made possible to form a thin layer, and the mechanical stress can be reduced at the gap portion.
In the developing device of Japanese Patent Application Laid-Open No. H10-333431, however, there is no description of stresses on the developer from the developer restricting members except the most downstream developer restricting member. Therefore, the degree or magnitude of the mechanical stress that can be reduced as the whole developer restricting unit is unclear. In addition, an adverse effect such that the number of parts is increased, that results in complication of the apparatus, and a manufacturing cost is increased is found in the developing device.
Japanese Patent Application No. 2002-258616 discloses a developing device in which excessive developer held by a developer carrier is caused to drop from a rotational center height of a developing roller due to its own weight to form a thinner developer layer by devising a magnetic constitution of the developer carrier, and a doctor blade does not perform restriction. In this developing device, since it is unnecessary to provide a doctor blade, mechanical stress on the developer can be reduced.
In order to perform development with a high quality faithful to an electrostatic latent image, toner in conveyed developer is charged sufficiently the developer is conveyed to a developing region. The toner in the developer is charged by contacting with carrier in a developer accommodating unit and it is further charged by contacting with the carrier under mechanical stress from a doctor blade to be conveyed to the developing region. For this reason, when mechanical stress due to the doctor blade is merely reduced like the apparatuses in Japanese Patent Application Laid-Open Nos. H8-278695 and H10-333431, it becomes impossible to charge toner in the developer sufficiently. The toner that is not charged sufficiently becomes easy to separate from the developer, which causes such a drawback as toner scattering or scumming. In this manner, two functions of achieving a long life of developer by reducing stress on the developer and ensuring charging performance of the toner are required for the doctor blade.
In the developing device that is not provided with a doctor blade, disclosed in Japanese Patent Application No. 2002-258616, a charging member that charges the developer and an averaging member that averages the developer reaching the developing region are required as separate members, that results in increase in cost due to increase of the number of parts.